It is generally known to use short, high-energy laser pulses for engraving marks, such as numbers or dates, on various surfaces, such as coloured paper or eloxated aluminium, by directing the laser beam towards a mask made of for instance copper or stainless steel and in which the desired pattern is removed. In this manner a minor portion, typically less than 10%, of the laser light passes through the openings of the mask and through an image-producing optical system usually including a lens, and is imaged on the surface of the article.
However, far the major portion of the laser light is reflected by the portions of the mask not being removed, and thus the process utilizes the efficiency of the laser only to a minor degree. The radiation reflected by the mask is furthermore damaging to the laser in case it is directed towards said laser, because reflected radiation can interfere with the operation of the laser or, in extreme cases, damage parts thereof. Therefore the mask is often made diffusely reflecting or absorbing on its surface facing the beam.
With respect to laser processing of highly reflecting materials U.S. Pat. No. 4,480,168 discloses the use of a mirror system for catching the reflections from the surface of an article situated immediately below the mirror system, and for redirecting said reflections to the surface of the article in order thereby to increase the coupling of light into the material. Thus the publication discloses a mirror system comprising a concave upper mirror with a small opening allowing passage of a focused laser beam, and a lower mirror with a reflecting surface facing the upper mirror and with a larger opening allowing the laser beam to leave the mirror system and radiate the surface of the article so as to be reflected therefrom back to the mirror system. The surface of the article forms thus an essential part of the mirror system for achieving an increased coupling of light into said surface. In other words the refections from the surface are redirected thereto by means of the upper mirror.
If the above system is to be used for laser-marking an article, the latter must according to the publication be situated very close to the lower mirror with the rather unfortunate sideeffect that the material removed by the marking squirts onto the edge of the larger opening and is deposited thereon. As a consequence thereof the opening geometry or dimension changes gradually.